The production of molded articles from foamed styrene polymers is well known. Such molded articles are typically manufactured by first heating expandable particles of a styrene polymer with steam to foam or expand the particles (preexpansion); allowing the preexpanded particles to "mature" for a certain period of time; introducing the preexpanded particles into a mold and heating same with steam; opening the mold after a specified cooling period, and then removing the molded article from the mold. Typically, the cooling period imparts the greatest influence upon the productivity of such process.
As noted above, the expandable particles, having been impregnated with a suitable blowing agent, are partially expanded or foamed prior to introduction into the mold for complete expansion. This preexpansion expands the particles to approximately one to six pounds per cubic foot in density from their previously unexpanded density of approximately forty-two pounds per cubic foot. The quality of the molded product is improved if the "preexpanded" particles do not form agglomerates during preexpansion. Various additives, such as zinc stearate and calcium polysilicate (ACP), are utilized to aid in the prevention of agglomeration.
After the particulate expandable styrene polymers are introduced into the mold and heated above the boiling point of the expanding agent and of the polymer softening point, the molded article must remain in the mold until the temperature drops below the softening point of the styrene polymer. Premature removal from the mold will result in articles that warp, collapse or shrink. This time required prior to ejecting the molded article from the mold is called the cooling time. Naturally, the shorter the cooling time, the faster that molded articles can be manufactured. Various coatings are therefore deposited on the surface of the particulate expandable styrene polymers in an attempt to shorten this cooling time, as well as prevent the agglomeration of particles during preexpansion.
U.S. Pat. Nos. 4,495,224, 4,238,570 and 4,189,515 disclose various methods for coating expandable styrene polymers. Included among such coating materials are fine-pored pulverulent inorganic compounds such as silicone dioxide, talcum, clay, magnesium oxide, magnesium hydroxide, magnesium carbonate, organic compounds such as waxes, metallic soaps such as magnesium or zinc stearate and esters of fatty acids or polyhydroxy compounds such as glycerin or sorbit esters. The use of such glycerin esters of long chain fatty acids is known in the art.
The above noted coating materials are deposited onto the surface of the particulate expandable styrene polymers by various methods. The objective is to obtain a uniform coating on the expandable polystyrene particles. When the coating material is a fine powder, a tumbling drum or dry mixing can be used, but uniform distribution of the coating material over the surface of the polystyrene particles is difficult to obtain. Uneven coating of the particles will lead to inconsistent processing of the molded article resulting in lower production rates.
Other attempts to obtain a uniform coating on the surface of the particulate polystyrene include the use of suspensions of the water insoluble glycerin esters in combination with an inorganic solid. U.S. Pat. No. 4,238,570 describes a method of coating expandable particles of polystyrene with an ester of an aliphatic carboxylic acid with an aliphatic alcohol which is a solid at room temperature, for example, a triglyceride of a fatty acid. According to the method disclosed therein, as compared with a triglyceride of a fatty acid which is a liquid at room temperature, the use of a solid ester reduces the blocking during preexpansion. Additional defects in the utilization of a liquid glyceride are set forth in the above-referenced patent. Further, two stated requirements of the coating composition disclosed in U.S. Pat. No. 4,238,570 are (1) that the ester have no hydroxyl group in the molecule; and (2) that the ester is a solid at room temperature (or a mixture of the ester in a finely divided lubricant). Examples of such esters include hardened beef tallow oil, hardened rape-seed oil, hardened fish oil or hardened wax.
Although the use of the above noted methods enhances the manufacturing process, it is desirable to have a process which produces a uniform coating on the surface of the particulate expandable styrene polymer and which reduces the cooling time required during the molding of articles from such polymers.